The steam heating of a paper sheet is widely practiced in papermaking. The increase in sheet temperature that results provides increased drainage rates for the water thus reducing the amount of water to be evaporated in the drier section. Water drainage is improved by the application of steam principally because heating of the sheet reduces the viscosity of the water, thus increasing the ability of the water to flow. Most of the heat transfer takes place when the steam condenses in the sheet. The condensation of the steam transforms the latent heat of the steam to sensible heat in the water contained by the sheet.
A particular advantage of steam heating of the paper sheet is that the amount of steam applied may be varied across the width of the sheet along the cross machine direction so that the cross machine moisture profile of the sheet may be modified. This is usually carried out to ensure that the moisture profile at the reel is uniform. Moisture measurement devices are well known in the papermaking art that can sense the moisture profile of a sheet of paper. If such an apparatus is scanned over the paper sheet, downstream of a steam distributor, then after measuring the water profile in the sheet, steam can be applied in varying amounts on a selective basis across the sheet, thus achieving the required uniform moisture profile at the reel.
A typical steam distributor is divided into compartments with laterally spaced-apart baffle plates that are covered with a partially perforated cover. Actuators supply steam to the compartments. By regulating the supply of steam into each compartment, it was possible to a limited extent to control the moisture profile of the sheet. Nevertheless, even with these improvements, the velocity of the steam passing through the perforated cover varies only with the actuator flow rates so ideal steam velocity cannot be achieved for different flow rates.